Category Archives: Java

A Spring Cloud Toy Project

Recently played with the Spring/SpringBoot/SpringCloud stack with a toy project: https://github.com/gonwan/spring-cloud-demo. Just paste README.md here, and any pull request is welcome:

Introduction

The demo project is initialized from https://github.com/carnellj/spmia-chapter10. Additions are:

  • Code cleanup, bug fix, and better comments.
  • Java 9+ support.
  • Spring Boot 2.0 migration.
  • Switch from Postgres to MySQL, and from Kafka to RabbitMQ.
  • Easier local debugging by switching off service discovery and remote config file lookup.
  • Kubernetes support.
  • Swagger Integration.
  • Spring Boot Admin Integration.

The project includes:

  • [eureka-server]: Service for service discovery. Registered services are shown on its web frontend, running at 8761 port.
  • [config-server]: Service for config file management. Config files can be accessed via: http://${config-server}:8888/${appname}/${profile}. Where ${appname} is spring.application.name and ${profile} is something like dev, prd or default.
  • [zipkin-server]: Service to aggregate distributed tracing data, working with spring-cloud-sleuth. It runs at 9411 port. All cross service requests, message bus delivery are traced by default.
  • [zuul-server]: Gateway service to route requests, running at 5555 port.
  • [authentication-service]: OAuth2 enabled authentication service running at 8901. Redis is used for token cache. JWT support is also included. Spring Cloud Security 2.0 saves a lot when building this kind of services.
  • [organization-service]: Application service holding organization information, running at 8085. It also acts as an OAuth2 client to authentication-service for authorization.
  • [license-service]: Application service holding license information, running at 8080. It also acts as an OAuth2 client to authentication-service for authorization.
  • [config]: Config files hosted to be accessed by config-server.
  • [docker]: Docker compose support.
  • [kubernetes]: Kubernetes support.

NOTE: The new OAuth2 support in Spring is actively being developed. All functions are merging into core Spring Security 5. As a result, current implementation is suppose to change. See:

Tested Dependencies

  • Java 8+
  • Docker 1.13+
  • Kubernetes 1.11+

Building Docker Images

In case of running out of disk space, clean up unused images and volumes with:

Running Docker Compose

Or with separate services:

Running Kubernetes

NOTE: Kubernetes does not support environment variable substitution by default.

Use Cases

Suppose you are using the kubernetes deployment.

Get OAuth2 token

curl is used here, and 31004 is the cluster-wide port of the Zuul gateway server:

Get organization info

Use the token returned from previous request.

Get license info associated with organization info

Use the token returned from previous request.

Distributed Tracing via Zipkin

Every response contains a correlation ID to help diagnose possible failures among service call. Run with curl -v to get it:

Search it in Zipkin to get all trace info, including latencies if you are interested in.
zipkin-1
zipkin-2

The license service caches organization info in Redis, prefixed with organizations:. So you may want to clear them to get a complete tracing of cross service invoke.

Working with OAuth2

All OAuth2 tokens are cached in Redis, prefixed with oauth2:. There is also JWT token support. Comment/Uncomment @Configuration in AuthorizationServerConfiguration and JwtAuthorizationServerConfiguration classes to switch it on/off.

Swagger Integration

The organization service and license service have Swagger integration. Access via /swagger-ui.html.

Spring Boot Admin Integration

Spring Boot Admin is integrated into the eureka server. Access via: http://${eureka-server}:8761/admin.
sba-1

Building & Debugging OpenJDK8 from Source on macOS High Sierra

While reading Understanding the JVM – Advanced Features and Best Practices, Second Edition (in Chinese) recently, there is a guide in chapter one to build JVM from source. It is based on OpenJDK7, which only works when using a Java6/Java7 VM as build bootstrap. Java8 bootstrap has more strict code checks and will finally fail the build. So, I just switched to a more recent OpenJDK8 code. The file name is openjdk-8u40-src-b25-10_feb_2015.zip.

The code provides a better build experience, and compiles on my Linux box almost out of box. But remember, do not use a gcc compiler >= gcc-6. It defaults to C++14 and breaks the build. On macOS, the build scripts seem only support gcc. Actually, a clang compiler is required to build the objc code.

1. So the first step after downloading and unzipping the code, modify the configure script:

Comment out the lines(2 appearances):

2. Now install freetype and run configure:

A slowdebug build disables optimization and helps a lot when debugging. The summary output looks like:

3. Apply the following patch to fix build errors. Partially picked from an official OpenJDK10 changeset:

4. Start the build:

Lots of warnings, but the build should finish successfully:

5. Debugging with lldb:

The output binary lie in openjdk/build/macosx-x86_64-normal-server-slowdebug/jdk. The output of java -version:

Never used lldb before, seems to be compatible with gdb:

Streaming MySQL Results Using Java 8 Streams

The article is inspired by the posts here and here.

There is a RESTful service as the infrastructure for data access in our team. It is based on Jersey/JAX-RS and runs fast. However, it consumes large memory when constructing large data set as response. Since it builds the entire response in memory before sending it.

As suggested in the above posts. Streaming is the solution. They integrated Hibernate or Spring Data for easy adoption. But I need a general purpose RESTful service, say, I do not know the schema of a table. So I decided to implement it myself using raw JDBC interface.

My class is so-called MysqlStreamTemplate:

  • It does not extend JdbcTemplate, since there is only one interface for streaming, not one series. I’m not writing a general purpose library.
  • It is MySQL only, I have no time to verify with other relation databases.
  • It does accept a DataSource as the parameter of the its constructor.
  • Staff like Hibernate session is not concerned, since it maintains Statement & Connection by itself.
  • Staff like @Transcational is not concerned, since we do not care about transactions. Actually, MySQL gives HOLD_CURSORS_OVER_COMMIT in StatementImpl#getResultSetHoldability() in its JDBC driver, saying that our ResultSet survives after commit.

So, here is my class. NOTE: closing our Statement & Connection requires explicit invoke of Stream#close():

Read inline comments for additional details. Now the response entry and controller mapping:

Complete code can be find on my GitHub repository.

My simple benchmark script looks like:

Dramatic improvements in memory usage as shown in jconsole, especially Old Gen:
all_memory
old_gen_memory

Some raw data from jmap:

  • Jersey
  • Spring Boot
  • Spring Boot with Streams

Setting up Hadoop HDFS in Pseudodistributed Mode

Well, new to the big data world.

Following Appendix A in the book Hadoop: The Definitive Guide, 4th Ed, just get it to work. I’m running Ubuntu 14.04.

1. Download and unpack the hadoop package, and set environment variables in your ~/.bashrc.

Verify with:

The 2.5.2 distribution package is build in 64bit for *.so files, use the 2.4.1 package if you want 32bit ones.

2. Edit config files in $HADOOP_HOME/etc/hadoop:

3. Config SSH:
Hadoop needs to start daemons on hosts of a cluster via SSH connection. A public key is generated to avoid password input.

Verify with:

4. Format HDFS filesystem:

5. Start HDFS:

Verify running with jps command:

6. Some tests:

7. Stop HDFS:

8. If there is an error like:

Just edit $HADOOP_HOME/etc/hadoop/hadoop-env.sh and export JAVA_HOME explicitly again here. It does happen under Debian. Not knowing why the environment variable is not passed over SSH.

9. You can also set HADOOP_CONF_DIR to use a separate config directory for convenience. But make sure you have the whole directory copied from the Hadoop package. Otherwise, nasty errors may occur.

OO Impelementation in Java

Continue with last article, we will try to write an identical application to use OO features including: encapsulation, inheritance, polymorphism, properties, meta info and event-driven mechanism. Java supports the 3 basic features in language level. It uses interfaces to implements event-driven. To implements properties and meta info, we have to write our own code. We want to implements API like someObject.setProperty(prop-name, prop-value). I write my own NewObject class:

To use our setProperty()/getProperty() method, all classes should derive from the NewObject class. To be consistent with the JavaBean convention, we assume that the getter/setter function to be “get”/”set” + capitalize_first_letter_of(member-variable-name).

Property annotation and PropertyAccess enum are defined to indicate properties:

ClassInfo and ClassInfoList annotation are defined to indicate class meta info:

Let’s see how to use them, our Base is defined as:

Since our implementation of properties are simply methods, they can be inherited by subclasses. But the class meta info cannot be retrieved in subclasses. They just get their own.

I do not want to demo events/listeners code here, just find them in source code in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestJavaObject-{date}.zip file.